Thin film composite (TFC) membranes including ultra-thin active layers and porous supports have been widely used in reverse osmosis (RO) or nanofiltration (NF) processes for desalination. In TFC membranes, the active layer controls separation properties, while the support layer enhances mechanical strength. The cross-linked aromatic polyamide produced by interfacial polymerization of m-phenylenediamine (MPD) and trimesoyl chloride (TMC) has been a successful commercial product. However, the aromatic polyamides making up the active layer are sensitive to chemical disinfectants such as chlorine, and the chemical attack by disinfectants ultimately results in membrane failure as measured by enhanced passage of both salt and water.
Chlorine is commonly used upstream from desalination process (RO/NF) to kill bacteria and other organisms. Since current desalination membranes with aromatic polyamide active layers degrade when exposed to chlorine, current installations require an extra process step to remove chlorine prior to RO. For municipal systems, the purified water then has to be re-chlorinated. These extra steps could be eliminated if the active layers were more tolerant to chlorine.
Additional coating layers have been applied on the aromatic polyamides, and/or the aromatic polyamides have been chemically modified to enhance chlorine resistance. In addition, new polymers without chlorine sensitive reactive sites (for example, sulfonated poly((arylene ether sulfone) polymers) have been proposed, and these systems have improved stability in the presence of chlorine agents. However, the membranes made from these polymeric materials have limited performance compared to conventional aromatic polyamides. In addition, some of the systems required difficult synthetic work or were difficult to fabricate, which increases membrane manufacturing costs.